Cathode structure



United States Patent O CATHODE STRUCTURE George A. Espersen, DobbsFerry, N. Y., assignor, by

mesne assignrnents, to North American Philips Company, Inc., New York,N. Y., a corporation of 'Delaware Application November 29, 1951, SerialNo. 258,875

6 Claims. (Cl. 313-240) The present invention relates to emissivecathode structures for electron discharge devices, and to a method ofsealing said structure. l

The principal object of this invention is to provide a cathode structureof high efficiency in which electrical leakage is minimized.

More specifically, it is an object of this invention to provide animproved cathode structure of the dispenscr type adapted for use inmagnetron tubes.

For a better understanding of this invention as well as other objectsand further features thereof, reference is made to the followingdetailed description thereof to be read in connection with theaccompanying drawing wherein the figure shows, in sectional view, amagnetron tube incorporating a cathode structure in accordance with theinvention.

In U. S. Patent 2,543,728 of Lemmen et al., there is disclosed adispenser type cathode wherein a reservoir of electron emissive materialis contained within a cavity in a tightly closed body of refractorymetal having a porous wall portion, the pores of which form the largestpassageways connecting the electron emissive material with the exteriorof the cathode. The present invention, in a preferred form, makes use ofa cathode structure of the dispenser type and further includes meanseffectively to seal the cathode structure. Provision is also made toreduce electrical leakage and to increase the emissive eficiency of thestructure. By way of illustration, a cathode in accordance With theinvention is inserted into a magnetron anode, but it is to be understoodthat the invention is not limited either to dispenser type cathodes orto the use of such cathodes in magnetrons.

Referring now to the figure, the major elements of the magnetron are acathode structure 10, an anode block 11 surroundng the cathodestructure, and pole pieces 12 and 13 disposed at either end of the anodeblock for producing a magnetic field extending longitudinally throughthe anode and transversely with respect to the electron stream betweenthe cathode and anode.

Extending through central bores in the pole pieces 12 and 13 are tubularmetallic sleeve members 14 and 15 respeetively, which serve to supportthe cathode structure centi'ally within the anode block. The sleevemembers are held coaXially with respect to the pole pieces by means ofinsulating spacei's 16 and 17 formed, for example, of Ceramic material.The anode block 11 is constructed to define a plurality of cavitiescircumferentially arranged about the cathode.

The cathode structure 10 is of the indirectly heated dispenser type andincludes a porous tungsten cylinder 18 fitted on a molybdenum tube 19provided with a constricted portion, thereby forming an annular cavity20 in the area between the cylinder 18 and the tube 19. This cavity isfilled with electron emissive material 21 such as barium carbonate. Thenature of the emissive material may be of the type described in theabove-mentioned Lemmens patent. The porous tungsten cylinder ispreferably fabricated in accordance with the teachings in the patentapplication of Levi, Serial No. 234,513, filed June 30, 1951. Extendingthrough the molybdenuin tube is a heater filament 22 coated with aninsulating layer, such as aluminum oxide, to prevent shorting on thecathode. The end leads 23 and 24 of the heater are extended throughsleeve members 14 and 15. The cathode assembly is completed byring-shaped caps 26 and 27,

2 preferably formed of molybdenum, which are forced on the lrespectiveends of the molybdenum tube 19 and are sealed to the ends of thetungsten cylinder 18 by means of washers 28 and 29.

The seals between the caps, 26 and 27 and the ends of the tungsten tube18 are of the diffusion type. The washers 28 and 29 are formed of ametal having a low melting point relative to tungsten and molybdenum,such as platinum, titanium, zirconium, tantalum or thorium. Thetechnique of sealing is as follows: The washers are placed between thesurfaces to be sealed and pressure is applied to the caps 26 and 27 bymeans, for example, of a clamp. Sufiicient pressure is applied to reducethe melting point of the Washer material so that diffusion into thewalls of the metals to be sealed takes place. In a preferred method thecathode to be sealed is held in a press during the diffusion sealingprocess. A moisture-free inert gas atmosphere is flowed about thecathode during the time that heat is applied to the entire assembly. Asthe temperature of the cathode is increased, the pressure against thejoints to be sealed grows greater due to thermal expansion of thecathode. The increased pressure on the washers lowers the melting pointthereof so that diffusion occurs. In place of a press, a jig typefixture similar to a C-type clamp may be used, the clamp being made ofinvar or other low expanson coeificient material.

It is well-known in the magnetron art that magnetron performance dependsin large part upon the secondary emisson characteristic (8); that is tosay the ratio of secondary to primary electron emisson. WithConventional cathodes of the dispenser type, this ratio is rather low,in the order of 1.5. For the purpose of increasing the value of thisratio, the outer surface of the tungten cylinder 18 is coated withmaterial 30 yielding a high secondary emisson coefiicient such asoxides, nitrates, and borides of thorium, zirconum, columbium, cobaltand other elements withstanding temperatures higher than 1270 C. andwhich do not decompose or evaporate When Operating at this temperature.The addition of any of the above-listed materials to the cathode surfacewill increase the value of the S by factors of from 2 to 20 fold, thusimproving cathode efficiency.

In order to reduce electrical leakage from the cathode, there aremounted on sleeve members 14 and 15, conically-shaped metallic shields31 and 32 which encircle the caps 26 and 27 of the cathode. The use of aconical shield increases the length of the path from the cathode sleeveto the rim of the Shield, thereby lowering thetemperature of the shield,particularly at the rim, where un- Controlled emisson occurs, whichemisson tends to raise the leakage current.

While there has been described what is at present a prepared embodimentof the invention, it should be understood that many modifications arepossible within the scope of the invention. For example, the cathode maybe of the directly heated type in which the caps of the cathode act aselosures, the interior of the porous tungsten cylinder being filled Withemissive material. Or, if preferred, the cathode may be heatedexternally by bombardment from a primer cathode. lt is intended in theannexed claims to cover all such changes and modifications as fallwithin the true spirit of the invention.

What is claimed is:

l. A cathode assembly comprising a cylindrical body having electronemissive properties and constituted by a cylindrical metallic base, aporous metallic tube surrounding said base, said base having aconstricted portion to form a cavity between said base and said tube,electron emissive material disposed in said cavity and a layer ofmaterial coating said body and having a relatively high secondaryemisson coefi'icient, supporting members connected to the respectiveends of said body, and a conically-shaped shield mounted on each-of-saidmembers for reducing electrical leakage, the month of said shield facingsaid body.

2. A cathode assembly comprising a cylindrical body having electronemissive properties and including a cylindrical metallc base, a poroustungsten tube surroundng said base, the ends of said base projectingbeyond the ends of said tube, said base having a constricted portiondefining a cavity between said base and said tube, ring-shaped' capsmounted on the projecting ends of said base, a washer interposed betweeneach cap and the adjacent end of the tube providing a ditfusion sealtherebetween, electron emissive material disposed` in said cavity' andalayer of secondarily emissive material coated on: the: surface of saidtube, supportingl members connected to the respective ends of said body,and a conicallyshapedf shi'eld' mounted on each of said members andencircling said caps.

31. A cathode assembly comprising a cylindrical body having el'ectron.emissive properties and' including av cylindrical metallic base, aporoustungsten tube surrounding said base, the end's' of said base extendingbeyond the ends of said tube, said? base having a constricted portiondefining a cavty` between said base andsaid tube, ringshaped capsmounted on the projecting ends of' said base, a washer interposedbetween each cap and the adjacent end=` of' the tube providing adiffusion seal therebetween, electron emissive material disposed in saidcavity and a layer of secondaril'y emissive material coated on thesurface of said tube, and supportng members connected to the respectiveends of said body.

4. The method of seal'ing a cathode assembly constiiuted by a molybdenumbase, a porous tungsten tube surrounding said base, the ends of saidbase projecting beyond' said tube and molybdenurn rings fitted on theprojecting ends of said base, comprising the steps of interposing awasher between each of said rings and the respective ends of said tubes,said washers being fabricated of a metal having a melting point which islow relative to tungsten and molybdenum, and applying pressure to therings toreduce the melting point of the washer material to an extenteffecting diffusion into the adjoining surfaces.

5. An assernbly as set forth in claim l wherein said layer is formed ofoxides of thorium.

6. An assembly as set forth in claim 1 wherein said layer is formed of'borides. of zrconium.

ReferencesV Cited in the file of this patent UNiTED STATES lATENTS1,267,827 Whitney May 28, 1918 2,121,589 Espe .inneV 21, 1938 2,131,204Waldschmidt Sept. 27, 1938 2,381,01`2 Stutsman Aug. 7, 1945 2,411,601Spencer Nov. 26, 1946 2,460,119 Blewett et al. Jan. 25, 1949 2,543,728Lemmens et al. Feb. 27, 1951

